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Exceedence
Flows |
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"Exceedence" is
a way to describe the percentage of time for which an observed
stream-flow is greater than or equal to a defined stream-flow.
Exceedence is used when stream-flow data are not normally
distributed (i.e. on a bell-shaped curve).
Most streams flows are not normally distributed because
high flow events can skew the data making the mean flow greater
than the median flow. |
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Low-flow events have high exceedence percentages, and
high-flow events have low exceedence percentages.
Low-flow events have a high exceedence percentage because most of
the time, observed flows exceed the low flow. Similarly, high-flow events
have low
exceedence percentages because most observed flows are lower than the
high-flow levels. |
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Figure 1. The mean monthly flow for the month of
March for a hypothetical stream. |
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In the
example illustrated by the Figure 1, a hypothetical stream has a
mean monthly flow in March of 45 cfs, and a median monthly flow
of 38cfs. The 50% exceedence flow in March is the median flow of
38cfs. This means that the median March flow 5 out of every 10
years will be greater than or equal to 38cfs.
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The stream has an 80%
exceedence flow of 25cfs. An
80% exceedence flow means that the median March flow will be
equal to or greater than
25cfs 8 years out of 10. |
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The
stream has an 10% exceedence flow of 75cfs.
An 10% exceedence flow means that the median March flow
will be equal to or greater than 75cfs 1
year out of 10. |
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The
algorithms proposed for establishing instream flows, future
allocations, and the floor flow are based on exceedence
values. |
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The
individual instream flows will be decreed as quantities in cubic
feet per second (cfs) as will the future allocation for non-DCMI
(non- domestic, commercial, municipal, and industrial) uses and
floor flows. The administrative provisions for these in stream
flows will, however, recognize they are being established based
upon estimated flow unless measured flows are available or
acquired. The provisions of the final decrees will provide a
mechanism for changes to these decreed amounts based upon actual
flows if such data become available. |
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To
prevent dewatering streams by future non-DCMI
use, future non-DCMI use would be curtailed at a floor equivalent
to the unimpaired monthly 80% exceedence flow. Consequently,
the flow values for the four categories will be as follows: |
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1.
For State and private basins, |
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instream
flows would be decreed for each month of the year at the 50%
exceedence level of the estimated unimpaired flow,
subordinated to a future non-DCMI use in the amount of 25% of the
lowest median monthly unimpaired flow value. |
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2.
For federal, non-wilderness basins, |
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instream
flows would be decreed for each month of the year at the 40%
exceedence level of the estimated unimpaired flow,
subordinated to a future non-DCMI use in the amount of 10% of the
lowest median monthly unimpaired flow value. |
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3.
For federal wilderness and Wild and Scenic basins, |
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instream
flows would be decreed for each month of the year at the 30%
exceedence level of the estimated unimpaired flow,
subordinated to a future non-DCMI use in the amount of 5% of the
lowest median monthly unimpaired flow value. |
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4.
The Special Areas |
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include
watersheds that hold special values including high value habitat
for fish resources, other special values, and areas where future
development opportunities would be preserved. The instream flows
and reservations for future non-DCMI use for the special areas
differ from the land based formula described above. |
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Special
Areas include: |
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Lower
Salmon River below Long Tom Bar to the mouth:
Instream
flows for the lower Salmon River downstream of the Wild and Scenic
Reach would be consistent with the application filed for the lower
Salmon River below Hammer Creek. The State application for the
instream flow in the Lower Salmon addresses the reach from the
mouth to Hammer Creek. The instream flows reach in the current
application will be extended to include the reach of the Salmon
below the Little Salmon. The instream flows in the reach between
the Little Salmon and the Wild and Scenic River will be based on
the downstream reach and adjusted for the inflow from the Little
Salmon River. The State instream flow will be made consistent with
the Wild and Scenic instream flow for the main Salmon River. |
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South
Fork Salmon River and tributaries contained within the Tribal
Priority Stream List:
Instream flows would be decreed for each month of the year at the
40% exceedence level of the estimated unimpaired hydrology,
subordinated to a future non-DCMI use in the amount of 5% of the
lowest median monthly unimpaired flow value.
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Upper Salmon:
The upper Salmon
basin includes a number of tributaries that meet the criteria of
“B” List streams. Instream flows established for the
tributaries or the mainstem Salmon will be in accord with Wild and
Scenic River instream flows and future allocations, subject to the
Order Approving Stipulation and Dismissing Objections in
Consolidated Subcase Nos: 63-25239, 75-13316, and 75-13606, issued
by Judge Daniel C. Hurlbutt, Jr., Presiding Judge, Snake River
Basin Adjudication, on June 16, 1998. |
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Lolo Creek:
Instream flows
will be decreed for each month of the year at the 40% exceedence
level of the estimated unimpaired hydrology, subordinated to a
future non-DCMI use in the amount of 10% of the lowest median
monthly unimpaired flow value.
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Bedrock
Creek:
Instream
flows will be decreed for each month of the year at the 40%
exceedence level of the estimated unimpaired hydrology,
subordinated to a future non-DCMI use in the amount of 10% of the
lowest median monthly unimpaired flow value.
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Upper
North Fork Clearwater River, Breakfast Creek:
Instream
flows would be decreed for each month of the year at the 40%
exceedence level of the estimated unimpaired hydrology,
subordinated to a future non-DCMI use in the amount of 10% of the
lowest median monthly unimpaired flow value.
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Future Uses for
“A” List streams.
The future use
allocations will provide water for non-DCMI uses. The parties will
study the overlap of existing uses and future use to determine if
additional criteria will assist the parties in allocating future
use. The goal is to avoid reducing streamflows to a level where
the unimpaired 80% exceedence value is the flow that the normally
occurs in the stream due to the combination of existing and future
use.
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